Deformation of a partially molten D′′layer by small-scale convection and the resulting seismic anisotropy and ultralow velocity zone

Abstract

Partially molten regions in the lowermost mantle have been inferred to exist by the discovery of the ultralow velocity zone (ULVZ). We consider a small-scale stagnant-lid convection in the D′′layer, following [Solomatov, V.S., Moresi, L.N., 2002. Small-scale convection in the D′′layer. J. Geophys. Res., 107, doi:10.1029/2000JB000063.], to investigate the relationship between partial melt and seismic anisotropies often found in the D′′layer. Such convection can bring melts in the D′′layer from the ULVZ, deform the melt inclusions, and accordingly would profoundly affect seismic structures including anisotropies. We therefore calculate the deformation history of partially molten regions at the base of the D′′layer which is heated from below, using a 2D model with a strongly temperature dependent viscosity. An initially isotropic partial melt is strongly deformed by the viscous stress caused by thermal convection, and becomes anisotropic by shape preferred orientation (SPO) of melt inclusions, whose aspect ratios are of the order of 1 0 1 to 1 0 2 at the base of the plume and become as large as 1 0 3 to 1 0 4 in the plume head. We calculate the effective elastic constants for such anisotropic media which contain deformed melt inclusions, and obtain the seismic velocity for a horizontal ray path. We find that the horizontally averaged velocity profile can be correlated with convective patterns. If melting occurs not only in the ULVZ but extends to the top of hot regions of the D′′layer, the vertical seismic profile consists of three layers corresponding to the base, conduit and head of a rising plume. The lowermost layer, which corresponds to the ULVZ, becomes strongly anisotropic with V SH > V SV . The deformation and alignment of the melt, rather than the melting itself, is primarily responsible for reducing the seismic velocity. On the other hand, in the conduit, the anisotropy is of V SV > V SH type because of vertical alignment. In the plume head, the anisotropy is of V SH > V SV type with a magnitude of about 2%. We discuss how shear wave anisotropy may be used to infer the temporal evolution of the D′′layer.